Molybdenum-iron-containing N2ase

The molybdenum-containing enzyme Mo-N2ase is the most well-studied, although all-iron, vanadium-containing and tungsten-containing enzymes are also known to exist. The Mo-N2ase enzyme is composed of two proteins, which are referred to as the iron protein and the molybdenum-iron protein [259-267]. 

The iron protein contains a single [Fe4S4] cluster, which mediates electron transfer to the FeMo-containing protein. The FeMo protein contains two 8Fe-7S clusters referred to as P-clusters and two lMo7Fe-9S clusters referred to as FeMo cofactors (FeMocos). 

The FeMoco is the putative active site for dinitrogen and proton reduction. 

The iron and molybdenum centers are extensively bridged by sulfide ligands. 

The mechanism of hydrogen production at FeMoco is not well understood. 

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The Mo enzyme consists of two components (1) the Fe protein (molecular weight 57,000 daltons), which contains iron and sulfur (4 atoms of each per protein) and (2) the MoFe protein (220,000 daltons, Uz subunits), which contains both metals (1 atom Mo, 32 atoms Fe). Each also contains S ions (ca. one per iron), which act as bridging ligands for the metals. The protein contains special Fe—S clusters called P clusters that have EPR resonances like those of no other Fe—S cluster. A soluble protein-fiee molybdenum and iron-containing cluster can be separated from the enzyme. This iron-molybdenum cofactor, or FeMo-co, was known to have approximately 1 Mo, 7-8 Fe, 4-6 S ", and one molecule of homocitrate ion. As for the P cluster, there was no agreement on the structure of FeMo-co for many years. In purified form FeMo-co does crystallize, and it can restore N2 reducing activity to samples of mutant N2ase that are inactive because they lack FeMo-co. On the other hand, no crystal structure of FeMo-co proved possible, and no synthetic model complex was found that could activate the mutant enzyme. 

Fe Protein. Characteristics of the Fe protein from Azotobacter 101) and Clostridia (94, 108) are summarized in Table VII. Clostridial Fe protein is stated to be 90-95% pure and the absence of tryptophan (102) suggests that almost all of the contaminating proteins have been removed. Recent preparations of Azotobacter Fe protein show specific activities even higher than those of clostridial Fe protein (101). The clostridial protein has a molecular weight of 39,000 and contains 2-3 Fe and 2 S atoms (94, 108). Molybdenum is absent, no ESR is detectable in the native protein (102) and, surprisingly, a resonance at g value of 1.94 has not been observed on reduction. All other purified iron-sulfur proteins with the exception of the atypical HIPIP and rubredoxin exhibit resonance in this area on reduction. One may suggest that the Fe protein is an atypical iron-sulfur protein or requires additional examination at 4°K for a resonance at g = 1.94. The individual protein has no activity alone but has all N2ase activities in combination with the Mo-Fe protein. 

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